A process for producing unsaturated aldehydes and/or unsaturated acids from C3˜C4 olefins by using a catalyst is a typical example of catalytic vapor phase oxidation.
Particular examples of catalytic vapor phase oxidation include a process for producing acrolein and/or acrylic acid by oxidizing propylene or propane, a process for producing methacrolein and/or methacrylic acid by oxidizing isobutylene, t-butyl alcohol or methyl t-butyl ether, or the like.
(Meth)acrylic acid, which is an unsaturated acid, is a material useful for various applications including coating agents, adhesives, plasticizers and various kinds of synthetic resins.
In general, catalytic vapor phase oxidation is implemented as follows. At least one catalyst in the form of granules is packed into reaction tubes 11, 12, feed gas 1 is supplied to a reactor through the reaction tubes and the feed gas is in contact with the catalyst in the reaction tubes to perform vapor phase oxidation. Reaction heat generated during the reaction is removed by heat transfer with a heat transfer medium, wherein the temperature of the heat transfer medium is maintained at a predetermined temperature. Herein, the heat transfer medium for heat exchange is provided on the outer surface of the reaction tubes to perform heat transfer. A reaction mixture 3 containing a desired product is collected via a duct and then sent to a purification step. Generally, catalytic vapor phase oxidation is a highly exothermic reaction. Therefore, it is very important to control the reaction temperature in a specific range and to downsize hot spots in the reaction zone.
To perform the partial oxidation of propylene, propane, isobutylene, t-butyl alcohol or methyl t-butyl ether (also referred to as ‘propylene or the like’ hereinafter), a multimetal oxide containing molybdenum and bismuth or vanadium or a mixture thereof is used as a catalyst.
Generally, propylene or the like is subjected to two-step catalytic vapor phase partial oxidation to produce (meth)acrylic acid as a final product. More particularly, in the first step 10, propylene or the like is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to produce (meth)acrolein 2 as a main product. In the second step 20, (meth)acrolein obtained from the preceding step is oxidized by oxygen, diluted inert gas, water vapor and an optional amount of catalyst to produce (meth)acrylic acid 3. The catalyst used in the first step is an oxidation catalyst based on Mo—Bi, which oxidizes propylene or the like to provide (meth)acrolein as a main product. Additionally, a part of (meth)acrolein is further oxidized on the same catalyst to form acrylic acid partially. The catalyst used in the second step is an oxidation catalyst based on Mo—V, which oxidizes (meth)acrolein-containing mixed gas produced in the first step, particularly (meth)acrolein, to provide (meth)acrylic acid as a main product.
Reactors for carrying out the above process are realized in such a manner that each of the above two steps are implemented in one system or in two different systems (FIG. 1).
Meanwhile, many attempts have been made to increase productivity of the reactor for producing acrylic acid by modifying the reactor structure, by suggesting an optimized catalyst for oxidation or by improving the operational conditions.
As mentioned above, vapor phase oxidation of propylene or the like, or (meth)acrolein is an exothermic reaction. Therefore, there is a problem in that a hot spot (a point whose temperature is abnormally high or where heat accumulation is relative high) is generated in a catalytic bed in the reactor. Such hot spots show a relatively high temperature compared to other parts of the reactor. Accordingly, in hot spots, complete oxidation proceeds rather than partial oxidation, thereby increasing production of by-products such as COx, and decreasing the yield of (meth)acrolein or (meth)acrylic acid. Further, exposure of catalyst to high temperature causes rapid inactivation of catalyst, thereby shortening the lifetime of catalyst. To solve these problems, a method for inhibiting generation of hot spots and equalizing the availability of catalyst over the whole reactor has been studied to obtain (meth)acrolein or (meth)acrylic acid with high yield and to use the catalyst for a long period of time. In this regard, many improved catalysts have been continuously suggested.
For example, Japanese Laid-Open Patent Nos. Sho43-24403 and Sho53-30688 disclose a method for packing a catalytic bed by diluting a catalyst with an inactive material in a stepwise manner from the inlet of feed gas to the outlet of feed gas. However, the above method has a problem in that it takes too much time and is very difficult to pack the catalytic bed while varying the dilution ratio with an inactive material from 100% to 0% gradually. In addition, Korean Laid-Open Patent No. 2000-77433 and Japanese Laid-Open Patent No. 2000-336060 disclose a method for using multiple kinds of catalysts formed by varying the kind and amount of alkali metals. However, the method has difficulty in producing catalysts having different activities at a correct ratio because the amount of alkali metal used therein is small.
Further, Korean Laid-Open Patent No. 1995-4027 discloses a method of packing a catalyst in such a manner that the catalyst particles have a relatively large size in the vicinity of a hot spot. However, in this case, such catalyst particles having a large size may cause occlusion of a reaction tube or may have insufficient activity. Thus, there is a problem in that it is not possible to obtain conversion of propylene or isobutylene and yield of (meth)acrylic acid to a desired degree.
Therefore, there is a continuous need to develop a method for producing unsaturated aldehydes and/or unsaturated fatty acids with high yield while permitting stable use of a catalyst, by controlling the temperature of the highest-temperature hot spot efficiently.